Mutation Analysis of TET2 Reveals the Clonal Nature of Refractory Anemia with Ring Sideroblasts

Abstract 1862

Refractory anemia with ring sideroblasts (RARS) is a myelodysplastic syndrome (MDS) characterized by isolated anemia, erythroid dysplasia only, less than 5% blasts and 15% or more ring sideroblasts in the bone marrow (2008 WHO Classification of Tumors of Hematopoietic and Lymphoid Tissues). The natural history of RARS is characterized by an initial phase of erythroid hyperplasia and ineffective erythropoiesis, which is usually stable for many years but in a proportion of patients may be followed by a phase of marrow failure, with or without the later emergence of leukemic blasts. Overall, RARS is a benign condition with a median survival of about 9 years (J Clin Oncol. 2005;23:7594-603). Since the vast majority of these patients have no cytogenetic abnormalities, the clonal nature of RARS has been questioned. However, a few studies of X-chromosome inactivation patterns performed in female patients have suggested that RARS derives from clonal proliferation of a multipotent hematopoietic stem cell with the potential for myeloid and lymphoid differentiation. Somatic mutations of TET2 have been recently found in myeloid neoplasms including MDS, where they appear occur early during disease evolution (Nat Genet. 2009;41:838-42), and are currently considered as a reliable clonal marker of these disorders. In this study, we therefore performed a mutation analysis of TET2 in patients with myeloid neoplasms associated with ring sideroblasts. Using direct sequencing, we studied 33 patients with RARS and 28 patients with refractory cytopenia with multilineage dysplasia (RCMD) having 15% or more ring sideroblasts in the bone marrow. Somatic mutations of TET2 were detected in circulating granulocytes from 10 out of 33 (30%) patients with RARS and 10 out of 28 (36%) patients with RCMD and ring sideroblasts. Most of these mutations were novel at the time of this writing. Fourteen patients had a single somatic mutation, and the mutation burden ranged from 10 to 80%. In 9 of these 14 cases, the mutation burden was approximately 50%, consistent with a fully clonal hematopoiesis characterized a single dominant clone that was heterozygous for the mutation. In a female patient with 10% mutant alleles, however, granulocytes carrying mutant TET2 represented only one tenth of clonal granulocytes as determined by X-chromosome inactivation patterns, suggesting the existence of alternative genetic events preceding the TET2 mutation and sustaining clonal dominance. Six patients had multiple somatic mutations of TET2: two mutations in 3 cases, three mutations in 2 cases, and four mutations in the last case. Quantitative evaluation of mutation burden showed concordant values (about 50%) for the multiple mutations in two patients (one with 4 and the other one with 3 somatic mutations of TET2), indicating the existence of a single dominant clone with multiple mutations. In the remaining 4 patients, discordant mutation loads were detected: the dominant mutation was present in about 50% alleles, while the remaining one(s) involved a lower proportion (10-35%) of alleles. These findings are consistent with the initial emergence of a clone of hematopoietic cells carrying a single mutation of TET2 and the subsequent development of subclones that carry additional TET2 mutations and become dominant with time. We also compared gene expression profiles of CD34-positive cells from patients with and without somatic mutations of TET2. While these 2 patient groups both had up-regulation of ALAS2 and down-regulation of ABCB7, distinctive “sideroblastic” features at the molecular level (Blood. 2006;108:337-45), no differentially expressed gene was identified between the 2 groups. These data indicate that somatic mutations of TET2 are unlikely to have a major impact on metabolic pathways at the CD34-positive cell level, and are more consistent with an epigenetic regulation function of TET2. In summary, this study shows that about one third of patients with RARS carry somatic mutations of TET2 in circulating granulocytes, clearly indicating that RARS is a true clonal disorder of hematopoiesis despite it presents as a benign erythroid disorder. In most cases, TET2 mutations appear to cause clonal dominance of hematopoietic stem cells, thus initiating the myelodysplastic process. During the clinical course of the disease subclonal evolution may occur through the acquisition of additional somatic mutations of TET2.